This invention relates generally to fuel cells, and more particularly to a system and method of inerting a fuel cell utilizing a wettable substrate associated with the anode and/or cathode of the fuel cell.
Fuel cell systems are electrochemical power sources for both stationary and mobile applications. One type of fuel cell employing a solid polymer electrolyte membrane (PEM) has shown particular promise as an automotive power source. That type of fuel cell includes a membrane/electrode assembly (MEA), with fuel, oxidant and coolant flow fields adjacent to the anode and cathode electrodes. The PEM fuel cells operate at relatively low temperatures, thus facilitating their application to automotive uses. Fuel cells do, however, have a characteristic that may present a drawback, particularly in automotive applications which typically require frequent shutdown and startup of the fuel cell.
Unless the fuel cell is in some way purged or otherwise inerted when it is being shutdown or even started up, undesirable processes and reactions may occur, or continue to occur. If the MEA of the fuel cell is not protected, as by inerting the anode and/or cathode flow fields by flooding with a purge gas, the electrochemical process may continue in some form and lead to undesirable results. Normally it is intended for the output voltage from the fuel cell to be reduced or terminated at shutdown, however the failure to inert may allow the process to continue in a way that depletes fuel volume on the anode side and creates a vacuum. If the system is not sufficiently leak tight, air may be drawn into the fuel side and lead to unwanted results.
It has been observed that the failure to adequately inert a fuel cell system may allow a fuel/oxidant (air) interface to exist, even if temporarily, at regions in the fuel cell, and such interface may lead to serious degradation in the performance and life of the cell. Indeed, even a fuel/nitrogen interface may have some deleterious affect on the fuel cell in this latter regard.
Although the desirability of inerting fuel cells is well known and has typically been accomplished through the use of an inert purge gas such as nitrogen, that particular process may detract from the overall economics and efficiency of fuel cell use. More particularly, the need to obtain, transport and/or frequently refill a source of inerting gas such as nitrogen is a significant obstacle to the acceptance and use of fuel cells as an automotive power source. To mitigate or avoid this obstacle, other techniques have been advanced as alternatives to the use of inert purge gas to inert fuel cells.
In one alternative, described in U.S. patent application Ser. No. 09/133,768, filed Aug. 8, 1998 for xe2x80x9cSelf-Inerting Fuel Cell Systemxe2x80x9d, and assigned to the assignee of the present invention, fuel, oxidant and coolant are individually controlled and caused to flow through respective flow fields in fine pore plates adjacent the anode and cathode sides of a membrane/electrode assembly in a PEM type fuel cell. Through selective control of the relative pressures of the fluids in the respective fuel, oxidant and coolant flow fields, coolant is permitted, at shutdown, to migrate through the fine pore plates and flood the fuel and oxidant (collectively xe2x80x9creactantsxe2x80x9d) flow fields, thereby displacing the reactants and inerting the fuel cell system.
While the aforementioned Self-Inerting system of U.S. Ser. No. 09/133,768 does provide a means of inerting a fuel cell without requiring the cost and inconvenience of a nitrogen purge, it relies solely on the mechanism of the coolant migrating through the fine pore plates and displacing (i.e., purging) the reactants. Although the mechanism of coolant migrating through fine pore plates may provide a speed advantage over a serial purge, especially in the instance of lengthy runs of serial, or serially-cascaded, reactant flow paths, such mechanism may nevertheless possess some limit in the speed at which it displaces the reactants, and that displacement mechanism is also its dominant, or even sole, mode of protecting the cell. Moreover, that mechanism may cause flooding of the reactant flow fields to a greater extent than is desired for efficient start up/re-start.
In view of the foregoing, it is an object of the present invention to provide a fuel cell inerting system which overcomes the drawbacks and disadvantages of prior fuel cell systems. Other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.
The present invention is a system for protecting fuel cells, especially of the PEM type, particularly during transient periods, such as during shutdown and/or start-up.
Accordingly, the present invention relates to an inerting system, both method and apparatus, for a fuel cell of the type that has a membrane/electrode assembly (MEA), which MEA includes anode and cathode electrode catalyst layers on respective opposite sides of the membrane, and a respective support plate comprising a respective substrate at at least one, and typically both, of the anode and cathode catalyst layers. In accordance with the invention, at least one of the support plate substrates is wettable, or hydrophilic. At least one fine pore plate is positioned adjacent a respective at least one wettable substrate, and serves to define a coolant flow field. A respective reactant flow field is also provided adjacent the wettable substrate, and is typically also defined by the fine pore plate, or by the wettable substrate in combination with the fine pore plate. The coolant flow field is spaced farther from the membrane relative to the reactant flow field. During operation of the fuel cell with an electrical load connected, and referred to as being xe2x80x9con loadxe2x80x9d, the reactant flow field is operated at a first pressure and the coolant flow field is operated at a second pressure lower than the first pressure to create a first pressure differential sufficient to prevent coolant from flooding the reactant flow fields and wettable substrates. However, during fuel cell shutdown, the reactant flow field is operated at a third pressure and the coolant flow field is operated at a fourth pressure, the fourth pressure being such, relative to the third pressure, to create a second pressure differential therebetween such that the coolant is caused, or allowed, to migrate through the respective fine pore plate and flood the respective wettable substrate to isolate the respective catalyst layer of the MEA from the respective reactant.
Each wettable substrate has small pores, typically 90% of the pores being between 20 and 50xcexc (microns), and is of relatively small volume compared to the fine pore plates, such that capillary forces may be utilized to facilitate the rapid transfer of coolant to the substrate under the appropriate pressure conditions.
In a representative arrangement, respective support plates are provided adjacent the respective anode and cathode sides of the MEA. Each support plate comprises a wettable substrate and, at least the cathode support plate, also a non-wettable diffusion layer. Moreover, there is a fine pore plate positioned adjacent each of the wettable substrates, with a fuel reactant flow field being positioned on the anode side and an oxidant reactant flow field being positioned on the cathode side, as well as each also having a coolant flow field. Each of those reactant flow fields operates at a pressure higher than the coolant flow field during on load operation of the fuel cell. During shutdown, the gas supplies to the reactant flow fields are stopped so that the flow fields come to ambient pressure, as may also be done with the coolant flow field.
If desired, an additional, supplemental inerting of the system may be obtained by controlling the coolant to also flood the reactant flow fields in accordance with the disclosure of the aforementioned Self-Inerting system of U.S. Ser. No. 09/133,768, which disclosure is incorporated herein by reference.
Provision may be made for purging the fuel flow field with air during fuel cell shutdown, after the fuel flow field has attained ambient pressure.
Moreover, the fuel cell may be started, or restarted, from the inerted shutdown state by setting the reactant and coolant flow fields at substantially equal pressures to flood, or maintain flooded, the wettable substrates; then purging the reactant flow field with the respective reactant until the flow field contains a homogeneous mixture of the respective reactant; and then operating the coolant flow field at a pressure level relatively lower than the pressure level of the respective reactant flow field to un-flood the respective wettable substrate.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.